This invention relates to a process for preparing L-fructose alone or in admixture with other L-sugars, such as L-mannose and L-glucose.
L-sugars are useful as sweetening agents because, as disclosed in U.S. Pat. No. 4,262,032, they are sweet like the D-sugars, but unlike D-sugars, L-sugars are either not metabolized by the body or are metabolized to a lesser extent than the D-sugars. These features make L-sugars desirable as sweeteners for individuals wishing to reduce caloric-intake or for individuals unable to metabolize common sugar sweetening agents without detrimental effects, e.g., diabetics. Another advantage associated with L-sugars include the absence of an objectionable aftertaste commonly experienced with artificial sweeteners such as saccharin and the cyclamates. However, as desirable as the L-sugars are in the foregoing respects, their relative scarcity in nature, particularly L-glucose and L-fructose, the laevo counterparts of the two monosaccharide sweeteners most commonly used today, has prevented their widespread use in foods and beverages.
Mayo, et al., "Enzymatic Preparation of L-fructose", Carbohyd. Res., 8 (1968), pp. 344-347, the contents of which are incorporated by reference herein, describes the enzymatic isomerization of L-mannose to L-fructose by employing L-mannose isomerase isolated from Klebsiella aerogenes cultivated in a nutrient medium containing L-rhamnose which is required to induce the microorganism to produce the enzyme. The need for L-rhamnose, a scarce and expensive L-sugar, imposes a considerable limitation on the practical usefulness of the Mayo, et al. process.